1. Field of the Invention
The present invention relates to illuminating apparatuses capable of uniformly illuminating objects to be illuminated or inspected, and relates to surface inspection systems including the illuminating apparatuses.
2. Description of the Related Art
To date, illuminating apparatuses for illuminating an object with light beams emitted from light sources have been used in image pickup apparatuses such as video cameras and digital cameras when the object side is in low light. Such illuminating apparatuses include a light source and optical components such as reflecting mirrors and a Fresnel lens that can efficiently guide light beams emitted from the light source forward (to the object side).
Among such illuminating apparatuses, some illuminating apparatuses can efficiently collect light beams emitted from a light source in many directions into a required illumination field angle.
Moreover, some illuminating apparatuses include optical members using total internal reflection such as a prism light guide instead of a Fresnel lens disposed in front of a light source so as to improve collection efficiency and reduce the size of the apparatuses.
Among these, an illuminating apparatus that illuminates an object with light beams emitted from a light source via a small optical member having a high collection efficiency is well known (see Japanese Patent Laid-Open No. 4-138438).
In the illuminating apparatus discussed in Japanese Patent Laid-Open No. 4-138438, light beams emitted forward from a light source are collected using a lens having a positive refracting power. Moreover, light beams emitted sideward from the light source are guided to a prism, and are reflected forward using total internal reflection. In this manner, light sources emitted in many directions are collected so as to illuminate the object side.
The illuminating apparatus discussed in Japanese Patent Laid-Open No. 4-138438 uses a small high-collectivity optical member capable of collecting light beams emitted from the light source in many directions and emitting the light beams from a common emergent surface.
In particular, the illuminating apparatus discussed in Japanese Patent Laid-Open No. 4-138438 can markedly increase the illuminance in the vicinity of the center of the object.
Moreover, Japanese Patent Laid-Open No. 8-262537 discusses an illuminating apparatus including an illumination optical system reduced in size by disposing a prism with a high collectivity in the anterior position to a light source, and having a light-emergent surface that emits light beams perfectly reflected inside the prism and is inclined with respect to an optical axis.
Moreover, Japanese Patent Laid-Open No. 8-234277 discusses an illuminating apparatus having an optical member using total internal reflection as a light guide located in the vicinity of a light source. The illuminating apparatus collects light beams emitted from the light source and converts the light beams into uniformly distributed light beams using the optical member so as to illuminate a surface with a small optical loss.
In general, illuminating apparatuses used with image pickup apparatuses such as cameras are required to uniformly and efficiently illuminate the object side.
Moreover, illuminating apparatuses used in surface inspection systems for detecting the presence of faults such as scratches, projections, and depressions on inspection surfaces of objects are required to uniformly and brightly illuminate the inspection surfaces of the objects.
In particular, stability in accuracy in detecting faults and durability of the systems are required for the surface inspection systems used for inspecting faults on the inspection surfaces of the objects. In order to improve stability in accuracy in detecting faults, it is useful to maintain sensitivity (accuracy) in detecting faults in the areas of the inspection surfaces of the objects at a constant level. Therefore, uniformity in illumination in the inspection areas is particularly important.
On the other hand, in order to increase the lifetime of, in particular, light-emitting tubes used for light sources, it is useful to efficiently illuminate the inspection surfaces with light beams emitted from the light sources.
In general, illuminance is increased by applying a higher voltage to the light-emitting tubes serving as the light sources, and this leads to a reduction in the lifetime of the light-emitting tubes. Therefore, illuminating apparatuses used in inspection systems for inspecting object surfaces are required to be used with low voltage (low illuminance), and to efficiently illuminate inspection surfaces so as to increase the lifetime of the light-emitting tubes and brightness of illumination.